1. Field of the Invention
The invention relates to the control of power in an FM signal.
2. Prior Art
The lnternational Telecommunications Union (Radio) (ITU-R) has published Recommendation 412. Among other things, this requires that the maximum power in the, frequency modulated (FM) stereo multiplex signal (including the pilot tone) shall not exceed the power produced by a sinewave that FM-modulates the carrier to a frequency deviation of xc2x119 kHz. This power is measured by integrating the signal, over a 60-second period. The exact wording is as follows:
2.5.1 For the radio-frequency protection ratios given in FIG. 1 and Table 3, it is assumed that the maximum peak deviation of xc2x175 kHz is not exceeded. Moreover, it is assumed that the power of the complete multiplex signal (including pilot-tone and additional signals) integrated over any interval of 60 s is not higher than the power of a multiplex signal containing a single; sinusoidal tone which causes a peak deviation of xc2x119 kHz.
NOTExe2x80x94The power of a sinusoidal tone causing a peak deviation of xc2x119 kHz is equal to the power of the coloured noise modulation signal according to Recommendation ITU-R BS.641, i.e. a coloured noise signal causing a quasi-peak deviation of xc2x132 kHz.
FM broadcasters throughout the world commonly use audio processing to reduce the peak-to-average ratio of the audio, improving the signal-to-noise ratio at the receiver and increasing perceived loudness for a given setting of a receiver""s volume control. An example of such a system is Optimod-FM Model 8200, manufactured by Orban, Inc.
Such systems usually combine a compressor, a high-frequency limiter, a peak limiter, and an overshoot-compensated filter. A stereo encoder (to create the standard xe2x80x9cpilot tonexe2x80x9d stereo baseband) may also be included in the system. Such a system is described in U.S. Pat. No. 4,103,243, and improvements thereon are described in U.S. Pat. Nos. 4,249,042 and 4,412,100.
Measurements have shown that it is not unusual for such a system to produce average power exceeding the ITU-R recommendation by 6 dB or more when the system modulates the FM carrier xc2x175 kHz, which is the typical 100% peak modulation limit. Indeed, measurements have shown that a significant number of commercial recordings exceed the ITU-R power limit-without any further audio processing.
It is therefore clear that some means must be found to meet the specification when processing arbitrary program material. Previously, this was done with an open loop, by defining presets for the audio processor that produce extremely conservative processing. By then testing the processor with these presets and known xe2x80x9cdifficultxe2x80x9d program material, one could have reasonable confidence that the processor""s output would not exceed the limits specified by the recommendation.
A method and apparatus for controlling the integrated multiplex power in a frequency modulated in (FM) processor is described. A power determining circuit determines the power in an input signal by squaring the signal. The result of the squaring is coupled, to a combining circuit, which combines the squared, signal with a threshold control representing the square of the maximum allowed power. This combined signal is integrated and then sampled. The results of the sampling are again integrated, clamped and used to control a compressor. The output of the compressor, after passing through a peak limiter, provides an output signal. Consequently, feedback is provided to the compressor through the loop comprising the squaring circuit, threshold control, first integrator, sample-and-hold circuit, second integrator and clamping circuit.